1. Field of the Invention
This invention relates to the manufacture of semiconductor substrates such as wafers and, more particularly, to improving the leveling and consequently the focusing of the substrate during the photolithography imaging procedure of the manufacturing process.
2. Description of Related Art
The manufacture of semiconductor substrates such as wafers and chips involve the use of high-resolution lithography systems. In such systems, the patterned mask (i.e., reticle) is illuminated with radiation (e.g., laser radiation or radiation from an arc lamp) that passes through the illumination system and achieves high-degree illumination uniformity over the illuminated portion of the mask. A portion of the radiation that passes through the mask is collected by a projection lens, which has an image field of a given size. The projection lens images the mask pattern onto an image-bearing substrate or workpiece such as a wafer. The workpiece resides on a workpiece stage that moves the workpiece relative to the projection lens so that the mask pattern is repeatedly formed on the workpiece over multiple “exposure fields.”
Lithography systems include an alignment system that precisely aligns the workpiece with respect to the projected image of the mask thereby allowing the mask to be exposed over a select region of the workpiece. Two types of lithography systems are typically used in manufacturing. One system is the step-and-repeat system, or “steppers” and the other is the step-and-scan system, or “scanner.” With steppers each exposure field on the workpiece is exposed with a single static exposure. With scanners, the workpiece is exposed by synchronously scanning the workpiece and the mask across the lens image field. An exemplary scanning lithography system and method is described in U.S. Pat. No. 5,281,996, which is incorporated herein by reference. The following description will be mainly directed to the step-and-scan system although it will be understood by those skilled in the art that the invention is applicable to any type imaging system.
As is well known, in a typical photolithographic process, a thin layer of a photosensitive material or photoresist is deposited over a semiconductor wafer. During the photolithography process, illumination such as ultra-violet light is illuminated through a lens system and a photolithographic mask or reticle to the semiconductor wafer. The reticle has a particular device pattern and the pattern is exposed over a portion of the wafer by the illumination to create exposed and unexposed regions on the wafer. These exposed or unexposed regions are then washed away to define circuit elements on the wafer. This photolithography process is repeated many times on different layers of the wafer to define many circuit elements on the wafer. At the end of the photolithography process, the wafer having an exposed device pattern is cut into semiconductor chips.
Typically, a reticle is made from a transparent plate and has a device exposure region and an opaque region. The plate is often made of glass, quartz, or the like and the opaque chrome region typically includes a layer of chrome. The device exposure region generally has a square or rectangular shape and is positioned in the center of the reticle. The device exposure region includes transparent portions and opaque portions defining a device pattern. The transparent portions in the device exposure region allow illumination from a light source to travel through them and reach the wafer. On the other hand, the opaque regions of the device region block the light and the light does not reach the wafer.
FIG. 1 shows a typical prior art reticle 80 having a square device region 82 surrounded by an opaque chrome region 84. For the sake of simplicity, a device pattern 88 in the device region is not illustrated in detail in the figure. There is a kerf region 86 at the periphery of the device region 82 between the device region 82 and opaque chrome region 84. The kerf region 86 typically contains important information regarding the photolithographic process of the wafer and usually includes test structures to verify the performance of a photolithographic process. For example, the kerf region may include alignment marks to check the accuracy of the reticle alignment and registration marks to measure the resolution of the device pattern during the photolithographic process.
Lithographic imaging is highly dependent on substrate uniformity. A lithographic process can tolerate a small range of topography variation through the “depth of focus” inherent in the process capability. However, unanticipated topography variation on the substrate is a known problem for lithography processes and can result in a faulty imaging process and a rejection of the imaged workpiece.
Modern exposure systems such as the step-and-scan exposure system utilize an optical lens leveling system to control the height (focus) of the scanning slit above the wafer. The exposure tool can adjust to fluctuations in step-height by a set of simple linear motions. The problem arises when major step-height changes occur across the reticle field as shown in FIGS. 2A and 2B. Even sophisticated leveling systems are faced with a conflict on where to place the imaging focal plane relative to the uneven topography and, in general, trade-offs are made in some form of minimizing the average focus displacement across the imaging field.
On most product chips the pattern density on any one layer is generally not uniform. This can lead to discrete topography patterns after processing several physical layers due to the response of polishing and other processes to the varying pattern densities. Lithography tools, which often sample the wafer topography before or during exposure, try to respond to the topography and sometimes in an undesirable fashion. A typical response can be a tilt in the focus plane relative to the wafer, which follows the overall topography of a chip, but can yield unsatisfactory results on both levels of topography. This problem is valid for both step-only and step-and-scan systems. Although with step-and-scan systems, the focus response of the image tool may also be affected in time as the tool scans over different levels of topography. In principle, topography effects in the scanning direction can be corrected by a positional movement of the wafer relative to the exposure apparatus, but this can potentially lead to either loss of throughput or degraded focus performance.
Further, in many cases, the pattern density in the test kerf is quite different than the product chips. Beyond this, product chips may also vary within the imaging field. As mask set costs escalate, it is increasingly common for customers to share these costs by coordinating a variety of different chips onto one reticle, sometimes even coordinating with other customers. Since these chips may have quite different design purposes, there is further opportunity for non-uniform pattern density. These pattern density offsets can eventually lead to a step height due to film application and CMP polishing variations over the different densities. Since the leveling spots of the leveling mechanism of the scanning exposure system are sampling different step heights, a stage (and focus) tilt is created as the slit scans over these areas. If the simple linear tilt leaves significant residual focus errors, these focus errors can cause critical failures in the product.
Such errors are difficult to predict, identify, and correct. Often, problematic step heights develop during back-end processing as additional layers build up on the chip. Identification of missing patterns attributable to focus errors in many of these levels can be challenging. In many cases, critical failures are not found with conventional in-line inspection techniques. Traditional fixes include improving the overall process latitude (often not feasible if step heights are excessive, or fixing the CMP/design issues that cause the step height). In any event, either of these are costly and time consuming.
U.S. Pat. No. 6,081,614 to Yamada et al. relates to a surface position detecting method applicable to a slit-scan type or scanning exposure type exposure apparatus, for continuously detecting the position or tilt of the surface of a wafer with respect to the direction of an optical axis of a projection optical system. As discussed therein, the focusing of a mask image in these apparatuses continuously performs corrective drive for auto-focusing and auto-leveling during the scanning exposure process. A level and surface positioning detecting mechanism uses an oblique projection optical system wherein light is projected to the surface of a wafer obliquely from above and wherein reflection light from the photosensitive substrate is detected as a positional deviation upon a sensor. From the measured values of level during the scans a corrective drive amount is made to the level (height) and tilt of the wafer as the measurement position passes the exposure slit region.
FIG. 1 of the patent is reproduced here as FIG. 5 and shown as a fragmentary and schematic view of a slit-scan type projection exposure apparatus to which a surface positioning detecting method of the patent is applicable. The figure of the patent is included herein for clarity to describe how the subject invention relates to a typical lithographic apparatus which provides a wafer level adjustment. As shown in FIG. 5, a reduction projection lens 1 has an optical axis AX and an image plane which is perpendicular to the Z direction. Reticle 2 is held by a reticle stage 3 and a pattern of the reticle 2 is projected by the reduction projection lens. Denoted at 4 is a wafer having a surface coated with a resist and 5 is a stage on which the wafer is placed. The wafer stage S comprises a chuck for attracting and fixing the wafer 4 to the stage 5, an X-Y stage moveable horizontally along an X-axis and a Y-axis direction, a leveling stage moveable along Z-axis direction (same plane as the AX direction) and also rotationally moveable about the X and Y axes and a rotatable stage being rotationally moveable about the Z axis.
Denoted as numbers 10–19 in FIG. 5, are components of the detection optical system for detecting surface position and tilt of the wafer 4. A light source is denoted as 10 and 11 is a collimator lens for transforming the light from the light source 10 into parallel light having a substantially uniform sectional intensity distribution. Denoted at 12 is a slit member of a prism-shape having a plurality of openings therein (typically five or six) to form level sensing spots 72 (39) on the wafer. Denoted at 13 is an optical system which serves to direct the independent light beams from the pinholes of the slit member 12 to independent measurement level sensing spots points on the wafer surface by way of a mirror 14. These are also called leveling spots or sensors as shown as number 39 in FIG. 3A and number 72 in FIG. 2A.
Next, a structure for detecting reflection light from the wafer 4 is shown by the structure elements 15–19. Denoted at 16 is a light receiving optical system which receives the light beams from the wafer surface 4 by way of a mirror 15. Stop member 17 is provided within the light receiving optical system 16 and the light beams that pass through the optical system 16 have their axes parallel to each other and are re-imaged upon a detection surface of a photoelectrically converting means unit 19 by means of separate correction lenses of a correction optical system unit 18.
Tilt correction of stage 5 (and hence wafer 4) is made so that the measurement (leveling) points on the wafer 4 surface and the detection surface of the photoelectrically converting means unit 19 are placed in an optically conjugate relation.
Main control unit 27 serves to control the whole system and provides output for the reticle position controlling system 22, surface position detection system 26 and wafer position controlling system 25. When the reticle stage 3 is scanned in the direction of arrow 3a, the wafer stage 5 is scanningly moved in the direction of an arrow 5a. As regards the alignment of the reticle pattern in a Z-axis direction, the leveling stage of the wafer stage is controlled through the wafer position controlling system 25 on the basis of the result of the calculation of the surface position detection system 26 that detects height data of the wafer 4. Specifically, height data related to height spots defined in the scanned direction and adjacent to the slit are calculated and the wafer is tilted in a direction perpendicular to the scan direction as well as the height with respect to the optical axis AX direction.
The Yamada et al. patent improves the wafer positioning by measuring beforehand errors to be produced with respect to the level detection points due to a difference in pattern structure among the level detection points disposed along the scan direction. The measurement error with respect to each level detection point is then used to correct the position of the wafer using the surface position detecting system.
Positional information of the reticle stage with respect to the X and Y directions is measured continuously by projecting laser beams from reticle interferometer system 21 to mirror 20. Likewise for positioning the wafer stage 5, using wafer stage interferometer 24, wafer positioning controller 25 and mirror 23.
Publication No. U.S.2003/0107719 to Chen also discusses the problem of focus variation in wafer fabrication. In this publication a method for correcting improper leveling tilt comprises determining the improper leveling tilt induced by a leveling sensor of a semiconductor equipment improperly detecting a semiconductor wafer having an asymmetrical semiconductor pattern as out of horizontal and applies a corrective leveling tilt to compensate for the improper leveling tilt induced by the leveling sensor.
In U.S. Pat. No. 6,172,757 to Lee a wafer level sensor apparatus is disclosed using an electrically adjustable, two-directional zoom lens. The adjustable zoom lens provides field-by-field alignment on a stepper by providing a variable field view and depth of focus on the viewed field.
As noted above, the subject patent application is applicable to any such leveling system.
All the above patents are hereby incorporated by reference.